This invention relates to a process for recovering ethylene oxide from aqueous solutions containing the same. More particularly, this invention relates to an improved process for recovering ethylene oxide from an impure aqueous solution further containing formaldehyde impurity and, optionally, acetaldehyde.
Ethylene oxide, a staple in commerce, is prepared by the industry in large quantities by oxidizing ethylene with air or elemental oxygen over a suitable catalyst, typically a silver-containing catalyst, at elevated temperature (100.degree. C. to 500.degree. C. is typical) and at superatmospheric pressure (2 to 25 atmospheres), e.g., by the process of U.S. Pat. No. 2,775,510.
The dilute ethylene oxide mixtures obtained from these reactions, which may be suitably conducted in fixed or fluid-bed reactors, are scrubbed in an adsorber with water to form an aqueous solution of ethylene oxide and to thereby separate the ethylene oxide from unreacted ethylene, oxygen and other gaseous components of the reaction mixture (e.g., carbon dioxide). The separated gaseous materials are generally recycled to the catalytic oxidation step. The aqueous ethylene oxide solution is withdrawn from the absorber and passed to a stripper, e.g., stripping column. In the stripper, generally steam is introduced, usually countercurrently to the ethylene oxide solution fed thereto, to remove ethylene oxide product as overhead. An aqueous stream containing small quantities of formaldehyde and ethylene oxide is withdrawn from the stripper as bottoms and is recirculated to the absorber for use in absorbing additional ethylene oxide.
The overhead product from the stripping column, containing CO.sub.2, ethylene oxide, gaseous inerts and water vapor, is cooled to partially condense the ethylene oxide and water contained therein, and the resulting mixture of vapor and liquid is passed to an ethylene oxide reabsorber, in which the uncondensed ethylene oxide vapor is reabsorbed in water. A predominance of the carbon dioxide and gaseous inerts which remain unabsorbed are readily separated as gaseous overhead stream from this reabsorption step. An aqueous solution is thus obtained which contains the reabsorbed ethylene oxide and aldehydic impurities, such as formaldehyde and acetaldehyde, as well as dissolved carbon dioxide and other gaseous impurities. This aqueous solution must be further treated to provide the high purity ethylene oxide required by the industry. In the process of U.S. Pat. Nos. 3,165,539, 3,174,262, and 3,964,980, this aqueous stream is passed to a "refining column" in which ethylene oxide is recovered as overhead and an aqueous bottoms is withdrawn for recycle to the reabsorber. In some processes (e.g., U.S. Pat. No. 3,904,656), the ethylene oxide overhead from the refining column is further purified in a second distillation column to remove any remaining carbon dioxide as overhead, and ethylene oxide bottoms are obtained which are passed to a third distillation column wherein purified ethylene oxide product is recovered as overhead.
The presence of high amounts of formaldehyde in the ethylene oxide stream can be undesirable from a processing standpoint. For example, the typical process of removing formaldehyde as an overhead bleed in the purification step following reabsorption of the ethylene oxide from the stripping column has several disadvantages. If the formaldehyde concentration in the overhead bleed is high, a solid paraformaldehyde phase can form in the overhead system of the column which can result in blockage and erratic operation and can possibly require shutdown and cleanout. See, e.g., J. Frederic Walker, Formaldehyde, pgs. 140-163 (3d Ed. Reinhold Publishing Corp.).
For the reasons outlined above, it is desirable to reduce formaldehyde content in the ethylene oxide stream as much as possible. It is further desirable to reduce formaldehyde content in ethylene oxide streams in as economical manner as possible.
U.S. Pat. No. 3,418,338 teaches a method wherein formaldehyde-rich streams produced during ethylene oxide purification can be further treated to reduce formaldehyde content by extractive distillation. However, separation of formaldehyde to increase the purity of the ethylene oxide product stream without the use of further distillation would be advantageous from a cost standpoint.
U.S. Pat. No. 4,134,797 to Ozero discloses a process for recovering ethylene oxide from an impure aqueous solution containing aldehydic impurities, wherein the impure solution is treated in a multi-stage, countercurrent distillation zone to separate the aldehydic impurities, resulting in an acetaldehyde-containing ethylene oxide stream, an ethylene oxide stream substantially free of aldehydic impurities, and a formaldehyde-containing ethylene oxide stream.
The Ozero patent reports that the distillate formaldehyde, i.e., the formaldehyde-containing ethylene oxide stream produced in the process therein contains usually from about 1000 ppm to about 3000 ppm of formaldehyde (see, e.g., column 9, lines 5-6). The ethylene oxide product stream is said to contain generally less than about 20 ppm and usually less than about 5 ppm of formaldehyde (see, e.g., column 8, lines 53-54).
Because certain applications are adversely affected by formaldehyde levels exceeding about 10 ppm, it is continually desirable to provide improved ethylene oxide purification methods which lower formaldehyde content to 10 ppm or less.
In the Ozero process, as is typical, formaldehyde is removed during distillation by washing the column with fresh, i.e., "once-through", water, which combines with the formaldehyde to form an "azeotrope" system and is removed with the bottoms product. Although Ozero uses the term "azeotrope" to describe the product formed from the combination of water and formaldehyde, it appears more likely that the water and formaldehyde react to form methylene glycol and polyoxymethylenes which alter the vapor-liquid equilibrium more than the water-formaldehyde "azeotrope". The term "apparent azeotrope" as used herein refers to the products formed from the combination of water and formaldehyde, including methylene glycol and polyoxymethylenes.
The formaldehyde-removing method disclosed in Ozero uses a great deal of water and leads to undesirable waste disposal problems. It is therefore desirable to provide an ethylene oxide purification process using distillation which reduces the formaldehyde content in the light ends stream and in the ethylene oxide product stream and uses less water.
While the methods described hereinabove produce an ethylene oxide stream which is substantially free of water, carbon dioxide and dissolved inert gases, these methods do not economically reduce the concentration of aldehydic impurities such as formaldehyde and acetaldehyde present in the ethylene oxide sought to be purified.
Therefore, one object of the present invention is to provide an improved process for purifying ethylene oxide which results in lower formaldehyde levels in the ethylene oxide product (e.g., 10 ppm or less) and in the light ends stream.
Another object of the present invention is to provide an improved distillation process for purifying ethylene oxide which uses less water to reduce formaldehyde content in the light ends stream and ethylene oxide product stream to the same levels achieved using the greater amounts of water as presently used.
These and other objects are achieved in the present invention.